refactor(langchain-classic): retarget deprecations to create_agent, other chores (#37164)

Sweep classic deprecations so every removal lands on `2.0.0`, runtime
warnings carry the auto-generated since/removal/alternative line, and
replacements steer at `langchain.agents.create_agent` and
`with_structured_output(...)` instead of pre-v1 LangGraph +
`python.langchain.com` links.

## Changes

- **Bump removal targets from `1.0` / `1.0.0` to `2.0.0`** across
agents, chains, memory, retrievers, structured-output, vectorstore
toolkits, and the `langchain_classic._api.module_import` shim — gives
users a real runway now that v1 has shipped.
- **Move bespoke `message=` strings onto `addendum=`** (or split into
`alternative=` + `addendum=`). `warn_deprecated` skips the
auto-generated since/removal/alternative line whenever `message=` is
set, so the prior pattern silently dropped that info from the runtime
`LangChainDeprecationWarning`. Matches the pattern already used in
`HTMLHeaderTextSplitter.split_text_from_url`, which is updated for
consistency.
- **Repoint `alternative=` at v1 replacements**: chains/memory/agent
toolkits → `langchain.agents.create_agent` (with checkpointer or
retrieval-tool guidance in the addendum); `openai_functions` and
`chains/structured_output` → `ChatModel.with_structured_output(...)`;
`openapi` chains → `ChatModel.bind_tools(...)` + HTTP client.
`ConversationChain` no longer points at `RunnableWithMessageHistory`.
- **Refresh `AGENT_DEPRECATION_WARNING`** in
`langchain_classic._api.deprecation` — drop stale LangGraph and
`python.langchain.com` links in favor of `langchain.agents.create_agent`
and the `docs.langchain.com/oss/python/migrate/langchain-v1` guide.
Propagates to all 13 caller sites in `agents/`.
- **Newly deprecate `langchain_classic.chat_models.init_chat_model` and
`langchain_classic.embeddings.init_embeddings`** with the framing
*"maintained in `langchain`; `langchain-classic` retains this entry
point for import-compatibility only"*. The classic docstring examples
and the warning admonition both point at `langchain.chat_models`.
- **Improve `init_chat_model` docstrings** in both `langchain_v1` and
the classic copy: clarify `provider:model` prefix vs. `model_provider=`,
recommend pinned IDs over moving aliases, add the `upstage` provider
row, and refresh examples to GA models (`gpt-5.5`, `claude-opus-4-7`).
- **Standardize partner Anthropic deprecations**: replace
`AnthropicLLM`'s `model_validator(raise_warning)` with
`@deprecated(since="0.1.0", removal="2.0.0",
alternative="ChatAnthropic")`, and pin the `ChatAnthropic`
`output_format` runtime warning at `langchain-anthropic 2.0.0` instead
of "a future version".

libs/partners/anthropic/uv.lock

@@ -510,7 +510,7 @@ wheels = [
 
 [[package]]
 name = "langchain"
-version = "1.2.15"
+version = "1.2.17"
 source = { editable = "../../langchain_v1" }
 dependencies = [
     { name = "langchain-core" },
@@ -734,7 +734,7 @@ wheels = [
 
 [[package]]
 name = "langchain-tests"
-version = "1.1.6"
+version = "1.1.7"
 source = { editable = "../../standard-tests" }
 dependencies = [
     { name = "httpx" },